11.6 Expressions

The internal representation for expressions is for the most part quite
straightforward. However, there are a few facts that one must bear in
mind. In particular, the expression “tree” is actually a directed
acyclic graph. (For example there may be many references to the integer
constant zero throughout the source program; many of these will be
represented by the same expression node.) You should not rely on
certain kinds of node being shared, nor should you rely on certain kinds of
nodes being unshared.

The following macros can be used with all expression nodes:

TREE_TYPE

Returns the type of the expression. This value may not be precisely the
same type that would be given the expression in the original program.

In what follows, some nodes that one might expect to always have type
bool are documented to have either integral or boolean type. At
some point in the future, the C front end may also make use of this same
intermediate representation, and at this point these nodes will
certainly have integral type. The previous sentence is not meant to
imply that the C++ front end does not or will not give these nodes
integral type.

Below, we list the various kinds of expression nodes. Except where
noted otherwise, the operands to an expression are accessed using the
TREE_OPERAND macro. For example, to access the first operand to
a binary plus expression expr, use: